They don't use filler per-say, but the pacing is REALLY slow. Instead of adapting 2-3 chapters per episode, they only do 1. They might even do half a chapter once in a while if Oda
takes a good amo un t o f breaks.
It's not THAT bad though, since Oda packs enough dialogue per chapter to keep it from feeling as slow as the OG Dragon Ball Z (another Toei show that did the 1 chapter-per-episode scheme, to more annoying results), but you can definitely tell they didn't expect the series to go on this long.